1. Field of the Invention
This invention relates to a gas flow valve and a sphygmomanometer air-feeding/discharging apparatus using the valve.
2. Description of the Prior Art
A conventional gas flow valve will be described with reference to the drawings. FIG. 10 is a perspective view of the appearance of a conventional gas flow valve. The gas flow valve shown in FIG. 10 is shown in FIG. 3 in the specification of Japanese Utility Model Application Laid-Open (KOKAI) No. 61-142003 filed by the present applicant.
In FIG. 10, a thin sheet-like member 30, which has a convex form and is called "flapper", functions as a body for regulating the amount of leakage of a gas flow. This convex thin sheet-like member comprises a substantially T-shaped thin piece made of plastic or the like base end of which has vanes 30a projecting to the left and right at right angles to the longitudinal direction of the member 30. Each of the vanes 30a has a shape and dimensions that enable the vanes 30a to fit into grooves 34 formed in a main body 32 of the gas flow valve. The longitudinal dimension of the thin sheet-like member 30 is such that, when the vanes 30a are respectively fitted into the grooves 34, the distal end of the member 30 will pass through a linear opening 38 provided in one end of the main body 32, as shown in the drawing.
When the thin sheet-like member 30 is mounted in an air passageway 36 of the main body 32, the linear opening 38 is opened so that gaps 40 are formed on both sides of the thin sheet member 30. As a result, venting takes place through the gaps 40. The main body 32 has a flat portion 32a having upper and lower surfaces serving as pressure-responsive surfaces so as to discharge air while changing the orifice area of each of the gaps 40 in response to a change in the cuff pressure acting on the flat portion 32a.
FIG. 4 shows a relation between the cuff pressure and the orifice opening area and the cuff venting ability. In the drawing, a characteristic curve B partially denoted by a broken line shows the orifice opening area of the above-mentioned conventional gas flow valve and shows that the orifice opening area substantially linearly changes within the range of cuff pressure from 250 mmHg at the highest to 0 mmHg.
A characteristic curve H denoted by a one-dot chain line shows the venting ability of a cuff made of an elastic material such as rubber or the like. As shown in the drawing, the venting ability rapidly decreases as the cuff pressure increases from 60 mmHg. This is attributed to the material and shape of the cuff.
On the other hand, a throttle valve is provided near the air-feeding outlet of a sphygmomanometer air-feeding bulb for the purpose of determining an appropriate pressure reduction rate with which the pressure is gradually reduced for measuring blood pressure during the reduction in cuff pressure. The throttle valve has a fixed orifice or a simple structure which enables the adjustment of the throttle valve. In the latter simple structure, the screw 100 shown by a two-dot chain line in FIG. 10 is screwed into a body (not shown) in order to enable the adjustment of the throttle valve, and the linear opening 38 of the gas flow valve 32, which is provided between the cuff and the air-feeding bulb, is pressed by the screw 100 in the transverse direction so that the minimum orifice area is set.
However, the conventional gas flow valve configured as described above includes a problem in that, when it is used in combination with a cuff having the venting ability shown by the characteristic curve H in FIG. 4, the venting rate, i.e., the pressure reduction rate, is rapidly decreased within the pressure range of from 60 to 0 mmHg, which yet allows measurement of blood pressure, and this makes constant venting impossible.
The throttle valve provided in a sphygmomanometer air-feeding/discharging apparatus using the conventional gas flow valve includes a problem in that, when it has a fixed orifice, as described above, it has no flexibility for cuffs having various shapes and specifications.
When the linear opening of the gas flow valve is pressed directly by the screw in the transverse direction for the purpose of regulating the throttle valve, there is a problem in that the orifice area is not constant, and thus the adjustment cannot be easily made. In addition, even if the pressure reduction characteristics of a constant rate discharge valve are adjusted to ideal state conditions which agree with the venting characteristics of the cuff connected to the valve, there is a problem in that the initial pressure reduction characteristics cannot be secured owing to differences in compliance with the cuff used.